The invention relates to a probe for measuring volatile components in an aqueous solution, for example to determine the alcohol concentration of an aqueous solution, with a probe body with a continuous lumen and a membrane disposed transversely with respect to the lumen, which isolates the lumen from the outside and with a semiconductor gas sensor disposed in a housing inside the lumen at a distance from the membrane forming a measuring chamber filled with air, said sensor responding to the gases permeating the membrane by changing its electrical resistance.
A probe for measuring organic solvents in a liquid using a semiconductor gas sensor in a closed measuring chamber with a liquid-impermeable membrane, through which the gases diffuse, is known from DE 31 26 648 C2. As the concentration of the measured components decreases, the gas enriched in the measuring chamber diffuses back through the membrane into the liquid. This probe however operates very slowly and is therefore unsuitable for use in industrial processes due to the very slow gas circulation by back-diffusion through the membrane.
Other probes for removal of volatile components from liquids or gases, for example to determine the concentration, and which operate with a permeation membrane as well as a sensor, are known for example from EP-A 0174 417 and EP-A 0054 537. In these known probes, permeation membranes based on a tubular silicone body are used, through which membranes the volatile components to be measured permeate at various rates depending on their concentrations and then reach the sensor, which, because of its electrical properties, changes its resistance according to the concentration of the components to be measured and emits corresponding measuring signals. To increase the measurement efficiency, the tubular permeation membrane is flushed with a carrier gas.
The disadvantage of these known probes is that, by using the tubular silicone membranes, which do not have substance-specific separating effects, not only is the desired substance/gas to be measured separated from the liquid and measured, but other gases and substances also distort the measurement result for a desired substance.
Moreover, the known silicone membranes have well-delimited diffusion rates, particularly with respect to alcohol, so that it becomes necessary to have a very large membrane surface area.
Thus, DE 196 04 606 A1 has already proposed a measuring device for one of at least two volatile components of a liquid, in which a permeation membrane composed of at least two layers with different permeation resistances and a carrier gas used in conjunction with a sensor.